When you look up a word, phrase, abbreviation, etc, in the Dictionary Dashboard widget, you will see a little half-circle to the left that contains the first letter of the item you looked up.
Clicking on that half-circle will bring up a list of related words, or words having the orginal word in them. For example, look up memory and then click on the "M" in the half-circle to see a list of words starting with memory bank, memory banks, etc. Clicking any of those words/phrases will jump to that entry in the dictionary. If you don't select a word, clicking the "M" again will return you to the definition of memory. I have used this semi-hidden feature a good many times now -- and it also works in Thesaurus mode.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070517200252331